The present invention concerns waveguide systems and devices.
Waveguides direct the propagation of light in a controlled fashion. A waveguide is therefore a fundamental component of systems and devices which depend upon the controlled use of light. The scale of waveguides in many modem devices is exemplified by hair sized optical fibers used in communication systems. In communications systems, such as telephone systems, the trend is toward use of optical signals and away from use of electrical signals. Practical reasons exist for the shift in focus to optically driven systems. Unlike electrical signals, optical signals are generally unaffected by electromagnetic fields created by such things as power lines and lightning. These sources of interference may create noise in electrical signals, but optical signals are unaffected.
Information capacity of optical signals is also potentially much larger than lower frequency electrical signals that are used in wired electrical and wireless electromagnetic communication systems. Generally, higher frequency signal carriers provide larger information capacity than lower frequency signal carriers. This is due to the wider bandwidth of the higher frequency signals. Another important benefit of communicating with optical signals is the aforementioned small size of optical fibers used as a transmission medium. A typical fiber having hair sized dimensions is a suitable replacement for bundles of copper wires having a much larger diameter. As demands for information access become larger and larger in modern times, the use of optical transmission systems places less demand on space in the construction of underground, above ground, and internal building communication systems.
Another important use of optical energy communicated through a waveguide is in cutting, weapons, and other high power laser technology. Laser light direct through a waveguide forms useful lasers for cutting everything from machine parts to patients undergoing delicate surgeries. Weapons technologies have focused on laser light as potential bases for systems that track and destroy projectiles, such as missiles, with the destruction being based upon energy from laser light.
Common difficulties are encountered in the practical implementation of such optical energy systems, however. Waveguides, e.g., fibers, introduce losses. Losses limit the distance by which the transmitter and receiver may be separated. These losses are generally referred to as optical signal attenuation. Absorption of signal light by the fiber acting as the transmission medium is one factor causing attenuation. Other factors leading to attenuation are the scattering of the signal light over a wider wavelength than the original transmission and radiative losses, typically occurring at bends in the optical fiber. Combination of these individual losses leads to a total signal attenuation characteristic for a particular optical transmission medium which is measured in decibels per kilometer.
An effect called Stimulated Brillouin Scattering (SBS) has been identified as a primary cause of scattering losses that limits the effectiveness of waveguides. SBS is an interaction of optical energy with acoustic energy. Optical energy guided into optical waveguides, e.g., the core of an optical fiber, produces acoustic energy. As is known in the art, once a certain amount of optical power is directed into a waveguide from another optical source or generated in the waveguide, the effect of SBS causes optical energy to backscatter into the source. Typical waveguides, e.g., optical fiber cable, are long enough (tens of meters) for the SBS interaction to be efficient at low signal power, and SBS is known to affect signals with spectral widths smaller than that of the SBS process. This backscattering is undesirable in most, if not all, applications.
Overcoming or reducing the SBS effect would therefore significantly impact many optical waveguide systems and devices. The ability to launch more power into an optical communication fiber, for example, has the alternative advantages of reducing the number of repeaters or, if the distance between repeaters is kept, of providing higher information capacity. In the field of work performing high power lasers, such as cutting lasers and weapons, overcoming the SBS effect offers the potential to use small semiconductor lasers. Though the semiconductor lasers have advantages in the area of power consumption and compactness, they have not yet found large application as work performing lasers due to the overall limited power developed by the lasers. Improving waveguide efficiencies would allow better use of the limited power developed and allow combination of separate powers from multiple lasers.
Currently, the highest brightness continuous-wave laser sources are fiber lasers and fiber-amplified laser sources. To realize, for example, a laser weapon, a high power laser for cutting applications, a high power free-space communications laser, a high power laser for tracking systems, or an earth-to-satellite power delivery system, multiple fibers can be combined to achieve required powers. However, the signal in each fiber must be coherent (narrow spectral width) enough to allow for beam steering and field shaping of the output of the fiber bundle over extended beam propagation distances. The result is that high-power fiber technology is limited by SBS to inadequate powers. Therefore, overcoming the SBS problem in optical fibers will open the doorway to a new generation of lasers and important applications.
Thus, there is a need for an improved method of limiting the SBS effect in waveguides. It is an object of the invention to provide such an improved method.
Those and other needs and objects of the invention are met or exceeded by the present method for reducing SBS in waveguides. The method of the invention controls the acoustic waves produced to be guided away from the portion of the waveguide which guides the light. The method of the invention results in novel systems and devices in which SBS effects are reduced and system efficiencies are increased.
In a preferred single clad optical fiber of the invention, cladding around the waveguide core of the fiber is set to guide the acoustic waves generated by the light which is guided in the core. Thus, acoustic waves are guided into the cladding. A substantial reduction in the SBS effect is then realized in the core that guides light.
The method of the invention is applicable to single clad optical waveguides, such as optical fibers, as well as dual clad optical fibers and other waveguides. A preferred dual clad (a.k.a. dual core) waveguide structure permits realization of a pump laser system having reduced SBS effect in the core used for guiding transmitted light and allows the light in the core to be pumped (amplified). A second core guides acoustic waves outside the core used for guiding transmitted light, and also guides pump light which amplifies the light transmitted in the core for light transmission. Pumping may also be assisted by cladding the second xe2x80x9cpumpxe2x80x9d core with a cladding that is anti-guiding for light and guiding for acoustic waves.